Answer: When Minnesota was surveyed, errors
were made due to the many lakes and swamps that the surveyors had to work
around.. The error that produced the Northwest Angle (the northern
most point in the lower 48 states) is probably the most famous.

Lake Wobegone's absence from Minnesota maps is due to another lesser
known error, but one that is well known to listeners of Garrison Keillor's
"The Prairie Home Companion", a weekly
radio variety show which is broadcast live, Saturdays, at 5:00 PM Central
Time by Minnesota Public Radio. When the first surveyors reached
Minnesota's western border, they were appalled to discover that their survey
of the state was one mile too wide. For expediency, their solution
for this "problem" was simple, they folded their survey map in the middle
to eliminate one mile and made the size of the state come out correctly.
Lake Wobegone is "in the fold", and it therefore does not appear on any
Minnesota map.